1. Field of the Invention
Biaxially oriented polyester films are used in packaging and in industry primarily where there is a need for their advantageous properties, i.e. good optical properties, high mechanical strengths, good barrier effect, in particular against gases, good dimensional stability when heated and excellent layflat.
For most applications it is also desirable, e.g. for reasons of effective presentation, to improve the optical properties of the polyester films, in particular the gloss and the film haze, while retaining good processability of the film. It is likewise desirable to improve the barrier properties of polyester films, e.g. in order to gain new applications.
2. Description of the Related Art
The prior art has demonstrated how it is possible to improve the optical properties, in particular the gloss and the haze, of biaxially oriented polyester films.
EP-A-0 514 129 describes a transparent multilayer film which comprises a substrate of a primary layer of polymer material which, at least on one of its surfaces, has a secondary layer of polymer material which has certain concentrations and certain size distributions of glass beads and silica particles. The secondary layer can be arranged on one or on both sides of the primary layer substrate. The haze and processing properties are improved in the film, but no improvement in the gloss and the barrier properties of the film are provided in the text. There is also no indication of any kind in the text as to how the topography of such a film should be adjusted for simultaneous improvement of gloss and oxygen barrier.
EP 0 604 057 describes a transparent multilayer film which comprises a substrate of a primary layer of polymer material which is essentially free from fillers and which, at least on one of its surfaces, has a secondary layer of polymer material which contains, as filler, in a concentration of from 100 to 1000 ppm, silicone resin having an average particle diameter of from 1.5 to 12.5 xcexcm. A disadvantage of the silicone particles is that these are comparatively expensive and do not provide an acceptable solution for the packaging market. In addition, films which are provided with pigments of this type tend to telescope more easily during reeling. In this text there is likewise no indication of any type as to how the topography of such a film should be adjusted for simultaneous improvement of gloss and oxygen barrier.
In many foodstuff packaging applications, there is demand for a high barrier effect against gases, steam and flavors (this having the same significance as low transmission or low permeability). A well known process for producing packaging of this type consists in high-vacuum aluminum metallizing of the plastic films used. Other well known processes consist in coating the films with oxidic materials (e.g. SiOx or AlxOy). Essentially, the coatings used are transparent.
The barrier effect against the substances mentioned above depends essentially on the type of the polymers in the film and the quality of the barrier layers applied. Thus very high barrier effect against gases, such as oxygen and flavors, is achieved in metallized, biaxially oriented polyester films. A barrier effect against steam is achieved in metallized, biaxially oriented polypropylene films.
The good barrier properties of metallized or oxidically coated films mean that they are used in particular for packaging foodstuffs and luxury foods, for which long storage or transport times create the risk that the packed foodstuffs become spoilt, rancid or lose flavor if there is an inadequate barrier; examples are coffee, snacks containing fats (nuts, chips, etc.) and drinks containing carbon dioxide (in pouches).
If polyester films metallized with an aluminum layer or having an applied oxidic layer are used as packaging material, they are generally a constituent of a multilayer composite film (laminate). Bags produced therefrom can be filled, for example, on a vertical tubular bag forming, filling and sealing machine. The bags are heat-sealed on their inward side (i.e. on the side facing the contents), the heat-sealable layer consisting generally of polyethylene or polypropylene. The composite film here typically has the following structure: polyester layer/aluminum or oxide layer/adhesive layer/heat-sealable layer. If the laminate thickness is from about 50 to 150 xcexcm, the thickness of the metal or oxide layer is only from 10 to 80 nm. Even this very thin layer is sufficiently effective to achieve adequate protection from light and very good barrier properties.
The oxygen barrier or the oxygen transmission is generally measured not on the laminate or the packaging itself, but on the metallized polyester film. To ensure good quality of the foodstuffs or luxury food even after relatively long storage times, the oxygen transmission (identical with permeability) of the metallized film may not be greater than 2 cm3/m2 bar d, but in particular not greater than 1.5 cm3/m2 bar d. In future, the demands of the packaging industry will head toward still higher barriers, with attempts to achieve permeability values of significantly less than 1.0 cm3/m2 bar d for metallized or oxidically coated films.
In the prior art, there is neither sufficient knowledge of the detailed basis for the barrier effect of metallized or oxidically coated polyester films nor of how this may be decisively improved. Variables which are clearly important are the area of the substrate and the type of substrate polymer and its morphology. It is generally assumed that smooth substrate surfaces result in better barrier properties.
In this connection, Weiss et al., in xe2x80x9cThin Solids Filmsxe2x80x9d 204 (1991), p. 203-216, studied the influence of the surface roughness of a substrate layer on the permeability. For this, polyester films were coated with lacquer which contained various concentrations of titanium dioxide particles. In the experiments described, the concentrations of titanium dioxide particles in the lacquer varied from 2 to 20% by weight. Using this method, the surface roughness Ra of the coated substrate surface could be varied from 43 nm (unlacquered and lacquered film, without titanium dioxide) to 124 nm. In his experiments, increasing roughness (increasing proportion of TiO2) of the lacquered surface resulted in markedly higher oxygen transmissions after metallizing with aluminum. However, the largest step increase in oxygen transmission was seen when the lacquered film (0% by weight) was compared with the unlacquered film, although the surface roughness of the substrate was the same in both cases. The lacquering alone of the film gave a deterioration in the barrier from about 0.43 cm3/m2 d bar (plain film) to about 19 cm3/m2 d bar (lacquered film). A further uncertainty concerning the transferability of this work to commercial products is created by the fact that the aluminum layer was applied using a laboratory evaporator. When compared with an industrial metallizer, this method achieves essentially low permeability values, and the influence of the substrate surface on the barrier properties cannot be seen clearly.
Other detailed results of studies on the influence of the substrate surface of polyester films on their barrier properties can be found in the dissertation by H. Utz (Technische Universitxc3xa4t Mxc3xcnchen 1995: xe2x80x9cBarriereeigenschaften aluminiumbedampfter Kunststoffolienxe2x80x9d [Barrier properties of aluminum-metallized plastic films]).
According to the studies by Utz (pp. 66 ff.), there is no direct correlation between the surface roughness (average roughness height Ra) of the PET film and its oxygen barrier. For example, the film for video applications which, with an average roughness height of Ra=22 nm, is highlighted as particularly smooth, has, at 1.3 cm3/m2 bar d, an oxygen transmission of 1.2 cm3/m2 bar d comparable with the much rougher PET II film (Ra=220 nm).
EP-A-0 124 291 describes a single-layer biaxially oriented polyester film for magnetic recording tape which has the following surface property parameters
a) the average roughness Ra is from 1 to 16 nm,
b) the coefficient of friction xcexck is from 0.01 to 0.20 and
c) the following relationship exists between Ra and xcexck
0.1 less than 10*Ra+xcexck less than 0.31. 
These properties are created by using TiO2 particles (anatase) or TiO2 and CaCO3 particles in a proportion by weight of, respectively, from 0.1 to 0.5% and from 0.1 to 0.3%. The diameter of the TiO2 particles is from 0.1 to 0.5 xcexcm. The surface of this film is formed by a large number of elevations/protrusions which obey a distribution such that the graph described by the following relationship
log y=xe2x88x928.0x+4.34, y greater than 10 
is not intersected. In this equation, x (xcexcm) is a height above a standard level and y is the number of elevations (number/mm2) if the elevations are sectioned at a height of x. The distribution of the elevations is determined using standard equipment for measuring roughness. According to this text, although the processing properties of the film are improved, no information is provided on improvement of the gloss, the haze and the barrier properties of the film. There is also no indication of any kind in the text as to how the topography of a film of this type should be adjusted for simultaneous improvement of gloss and oxygen barrier.
EP-A-0 490 665 A1 describes a single-layer biaxially oriented polyester film for magnetic recording tape; the film contains
a) from 0.05 to 1.0% by weight of xcfx89-alumina having an average particle diameter in the range from 0.02 to 0.3 xcexcm, and
b) from 0.01 to 1.5% by weight of inert particles of a type other than xcfx89-alumina and having an average particle diameter in the range from 0.1 to 1.5 xcexcm, these particles being larger than the xcfx89-alumina particles.
The surface of this film is formed by a large number of elevations/protrusions which are described by the relationship
xe2x88x9211.4x+4 less than log y less than xe2x88x9210.0x+5 y greater than 30, x greater than 0.05 xcexcm 
In this equation, x (xcexcm) is a height above a standard level and y is the number of elevations (number/mm2) if the elevations are sectioned at a height of x. The distribution of the elevations is measured as in EP-A-0 124 291. This text, like that mentioned above, gives no information concerning improvement of the gloss, the haze and the barrier properties. It also gives no indication of any type as to how the topography of such a film should be adjusted for simultaneous improvement of gloss and oxygen barrier.
The prior art also discloses films which have different roughnesses on their two surfaces (dual surface). These films are suitable in particular for magnetic recording media and essentially have different topographies (e.g. surface A smooth, surface B rough). These texts generally provide means of improving the processing properties of the film but not its optical properties. In particular, however, these texts do not provide any means of improving the barrier properties of the film.
DE-A-16 94 404 describes a layered material having more than one layer of an oriented crystallizable thermoplastic film and in which at least one of the outer layers contains an additive. The additives are customary inert inorganic or organic particles, and in the case of inert particles such as SiO2, are added to the outer layers in concentrations of from 1 to 25% by weight, the particle size being from 2 to 20 xcexcm. The layered materials may, for example, be metallized with aluminum for decorative uses or used for magnetic tape. Although this text provides a means to improve the processing properties and the haze of the film, it does not provide a means for improving the gloss and the barrier properties of the film. The text also gives no indication of any type as to how the topography of such a film should be adjusted for simultaneous improvement of gloss and oxygen barrier.
DE-A-22 30 970 describes a magnetic recording medium which is composed of a biaxially oriented polyester film and a thin magnetic metallic layer on the surface A of the polyester film. The film comprises
a) a coated surface A, which is free from particles and
i) is at least 4 xcexcm thick or
ii) makes up at least 50% of the thickness of the entire film layer; and
b) a second layer containing particles and having relatively rough surface and containing
i) at least 1% of individual particles of a particular polymer A and
ii) at least 1% of individual particles of a particular polymer B.
A disadvantage of the film is that the surface A tends to block, so that the film does not process well. The text does not disclose a means for improving the gloss, the haze and the barrier properties of the film. Once again, there are also no indications of any type in the text as to how the topography of such a film should be adjusted for simultaneous improvement of gloss and oxygen barrier.
EP-B-0 061 769 describes a magnetic recording medium which is formulated from a biaxially oriented polyester film and a thin magnetic metallic layer on the surface A of the polyester film. If desired, there is also a lubricant layer on the other surface B of the polyester film. Features of the film are that the coated surface A
a) has an average roughness Ra (peak-valley value) of not more than 5 nm (60 nm),
b) the number of protrusions having a height of from 0.27 to 0.54 xcexcm is from 0 to 0.2 per mm2 and
c) is free from protrusions having a height greater than 0.54 xcexcm.
A disadvantage of the film is that the surface A tends to block, so that the film does not process well. The text gives no teaching on improvement of the gloss, the haze and the barrier properties of the film. There is also no indication of any type in the text as to how the topography of such a film should be adjusted for simultaneous improvement of gloss and oxygen barrier.
EP-B-0 088 635 describes a coextruded biaxially oriented polyester film having at least two layers, of which a layer A consists of thermoplastic resin and a layer B comprises thermoplastic resin and fine particles. The surface roughness Ra of the outer layer on the layer A in the film is less than 5 nm and the outer surface of the layer B is either
i) a surface having a surface roughness Ra of from 5 to 40 nm and a large number of depressions and a large number of protrusions which are arranged in a particular arrangement or
ii) a surface which has protrusions formed on a level area and which is covered by a layer C, which consists of a lubricant and has a surface roughness Ra of from 5 to 40 nm.
A disadvantage of this film surface is that it blocks, both with itself and with certain other surfaces (e.g. rubber rolls). The film cannot be processed cost-effectively; in particular during metallizing in vacuo, this film, because of its high tendency to block, tends to tear, and this can cause great cost problems. The film is unsuitable for the purposes of the object to be achieved. In addition, the haze of the film is unsatisfactory.
EP-B-0 502 745 describes a coextruded biaxially oriented polyester film having at least three layers, of which an outer layer A
a) contains inorganic particles having an average primary particle size D in the range from 1 to 100 nm and satisfying the equation D less than T less than 200D, where T is the thickness of the layer A and
b) contains particles B having an average primary particle size D1 in the range from 0.3 to 2 xcexcm, where the primary particle size distribution has a coefficient of variation of not more than 0.6 and
c) the average primary particle size D of the particles A is smaller than the average primary particle size D1 of the particles B.
The processing behavior of the film, in particular, is improved by applying the teaching of this text. The text does not give any teaching on improving the gloss, the haze or the barrier properties of the film. The text also gives no indication of any type as to how the topography of such a film should be adjusted for simultaneous improvement of gloss and oxygen barrier.
It is also known that the oxygen barrier can be improved by selecting particular polymers for the film serving as substrate (Schricker, G.: Metallisierte Kunststoffolien fxc3xcr hxc3x6herwertige Verpackungen [Metallized plastic films for high-quality packaging] in: ICI 5th International Metallizing Symposium 1986, Cannes). Polyesters, for example, are particularly suitable, specifically those made from ethylene glycol and terephthalic acid or from ethylene glycol, terephthalic acid and naphthalene-2,6-dicarboxylic acid. Besides these, polyamides, ethylene-vinyl alcohol copolymers (EVOH) and polyvinylidene chloride may also be used with advantage. Thus, for example, U.S. Pat. No. 5,506,014 describes a copolyester made from (a) from 45 to 85 mol % of terephthalic acid, (b) from 10 to 40 mol % of naphthalenedicarboxylic acid and (c) from 5 to 15 mol % of a dicarboxylic acid having from 2 to 8 carbon atoms and (d) ethylene glycol; (the molar percentages are based on the total proportion of dicarboxylic acids). This polyester is claimed to have better barrier properties against gases. It is used, inter alia, for producing bottles or containers and films of various thicknesses. A disadvantage of the raw materials mentioned is that they are significantly more expensive than polyethylene terephthalate (PET) or are unsuitable and/or not officially permitted for use in the packaging of foodstuffs.
It has been an object of the present invention to provide a coextruded biaxially oriented polyester film which has very good optical properties, i.e. low haze and in particular high gloss, at least of one film surface (termed film surface A below). A surface A of this type would, for example, be particularly advantageous for printing or metallizing. The high gloss of the film is transferred to the print and to the metal layer and thus ensures a particularly brilliant appearance of the packaging. This applies both to reverse printing and also to top printing of the film. In the production of stamping foils, in which the metal layer is transferred from the polyester film onto a carrier made from, for example, board, paper or plastic, the high metallic gloss is retained on the carrier and gives the transfer print an appearance which is particularly effective for presentation purposes.
At the same time, the film should exhibit a high oxygen barrier after metallizing or oxidic coating of the surface A. It should moreover be easy to produce and to process. In summary, the object was to provide a film having the following combination of features:
high gloss, in particular of the surface A of the film
low haze
low oxygen permeability of the film after metallizing or oxidic coating of the surface A of the film
low coefficients of friction; (no blocking of the film with itself).